As a malt alcoholic drink such as beer and low-malt beer is poured into a vessel at the time of drinking, a foam layer is created on the surface of the malt alcoholic drink. The property of the foam layer is one important factor indicating the quality of the malt alcoholic drink.
The property of the foam layer is represented by the degree of foaming, the thickness of the foam layer, the continuity of the foam layer (foam retention), the size of the particle diameters of foam, and the adhesive property of foam to the vessel, etc., and is a parameter for evaluating each foam layer objectively.
With respect to the particle diameter of foam, a foam layer composed of fine and uniform foam particles is called as creamy, continuous, and high quality foam to the eye. On the other hand, large and coarse foam particles that are created immediately after pouring into a vessel and collapse rapidly on the upper layer in a foam layer have no continuity. When such large and coarse foam particles are relatively abundant, the overall continuity of the foam layer is small and it is not a creamy foam. Accordingly, the property of created foams can be determined by measuring the particle diameter of the foam. The measure of the particle diameter of foam not only determines the property of foam on a malt alcoholic drink such as beer and low-malt beer, but also can be applied to determine whether or not very fine and creamy foam particles have been created, for example, in a foam layer of whipped cream.
However, no measuring method capable of appropriately measuring the particle diameter of foam on malt alcoholic drinks has been established and the measure of the particle diameter of foam has to depend on visual evaluation in the present circumstances, so that it is difficult to obtain an accurate measurement of the particle diameter of foam.